1. Field of the Invention
The present invention relates to a receiver having an AGC (Automatic Gain Control) circuit.
2. Description of the Related Art
In a receiver having an AGC circuit, when it is receiving the broadcast near the upper edge of the MW (Medium Wave) band (i.e. not less than 1500 kHz), there arises a problem that the receiving disturbances are generated due to the receiving interference and the degradation in sensitivity.
One example of such a receiver having the AGC circuit is shown in FIG. 7A.
In FIG. 7A, a resistor Rs (=80.OMEGA., for example) and a capacitor Cs (=80 pF, for example) represents an antenna impedance in the MW band, and a VIN represents an antenna induced voltage. The receiver is provided with: a LPF (Low Pass Filter) coil L.sub.1 (=several tens .mu.H, for example) for cutting off the frequency not less than the SW band; a HPF (High Pass Filter) coil L.sub.2 (=ten plus several mH, for example) for cutting off the frequency not more than the LW band; and a FET (Field Effect Transistor) A. A capacitor Cgs (=ten plus several pF, for example) represents the total of an input capacitance and an stray capacitance of the FET A. An AGC circuit is constructed by a capacitor Cagc (=several thousands pF, for example), and diodes D.sub.1 and D.sub.2, such that the AGC is operated when the operative resistances Rd of the diodes D.sub.1 and D.sub.2 are decreased. An electric current source Iagc changes the operative resistances Rd of the diodes D.sub.1 and D.sub.2.
The inventor of the present application conducts a simulation as for the frequency characteristic of the receiver of FIG. 7A, and the result of the simulation is shown in the graph of FIG. 7B.
In the graph of FIG. 7B, when the AGC is not operated and thus each of the operative resistances Rd of the diodes D.sub.1 and D.sub.2 is 1 M.OMEGA., a peak P2 is located at the vicinity of 8 MHz by virtue of the LPF coil L.sub.1 and the capacitor Cgs. When the AGC is sufficiently operated and thus each of the operative resistances Rd becomes several tens .OMEGA. e.g. 20.OMEGA. by virtue of the electric current source Iagc, the peak P2 is moved downward to a peak Q2 located at the vicinity of 3 MHz (or not more than this value) since the composite capacitance of the capacitor Cs and the capacitor Cagc becomes dominant. Since a slope B of this peak is positioned at the vicinity of the upper edge of the MW band (not less than 1500 kHz), the phenomenon that the AGC range runs short (i.e. the AGC becomes difficult to be operated) occurs, and the aforementioned problem is raised. In the above explained simulation, the frequency is the result of the calculation and the actual frequency is slightly higher than that in the simulation.
In this manner, the inventor of the present invention has found the fact that the cause of the problem (e.g. the receiving disturbances due to the receiving interference and the degradation in sensitivity) is such a frequency characteristic that the operation range of the AGC circuit of the MW receiver is varied depending on the frequency.
In order to overcome this problem, it may become necessary that the peak frequency at which the AGC is operated, is set to a frequency which is at least several times as high as the receiving frequency.
As one solution, it may be proposed that the LPF coil L.sub.1 is made small. However, in this case, the property to remove the unnecessary signals (e.g. FM signal, TV signal), which are unnecessary for the MW receiver having the frequency band not less than the SW band, is deteriorated.
Alternatively, it may be proposed that the capacitor Cagc is reduced. However, in this case, since the AGC operative range is determined by the ratio of the capacitors Cs and Cagc, the AGC operative range cannot be maintained as it is.
Here, it is to be noted that the capacity Cs is equivalent to the capacity of the antenna, and it is not possible to change this value by the design of the circuit.
In this manner, it is rather difficult to improve the receiver 70 shown in FIG. 7, by changing the parameters.
There is another receiver having the AGC circuit to overcome the above mentioned problem, which is shown in FIG. 8A.
In FIG. 8A, the same elements as those in FIG. 7A, carry the same reference numerals and the explanation thereof are omitted. The difference between a receiver 80 of FIG. 8A and the receiver 70 of FIG. 7A, is the position of the LPF coil L.sub.1.
The inventor of the present application again conducts a simulation as for the frequency characteristic of the receiver 80 of FIG. 8A, and the result of the simulation is shown in the graph of FIG. 8B.
In the graph of FIG. 8B, a peak P3 is located at the vicinity of 8 MHz when each of the operative resistances Rd of the diodes D.sub.1 and D.sub.2 is 1 M.OMEGA., and the peak P3 is only slightly moved to a peak Q3 when the AGC is sufficiently operated (i.e. when the operative resistance Rd becomes 20.OMEGA.), so that the problem of the aforementioned receiver is solved. Namely, the peak Q3 in FIG. 8B is shifted to the higher frequency side by about 2 MHz as compared with the peak Q2 in FIG. 7B.
However, the receiver 80 of FIG. 8A still has following problems, since the capacitor Cagc and the diodes D.sub.1 and D.sub.2 are directly connected to the antenna.
Namely, it has a first problem that the capacitor Cagc, and the diodes D.sub.1 and D.sub.2 generate a distortion of the received signal for the higher frequency range outside of the MW band. This distortion becomes the cause of the receiving disturbance upon receiving the MW band, and also the receiving disturbance upon receiving the FM signal which is not shown in FIG. 8B, for example.
Since the capacitor Cagc becomes a low impedance in the band range from the FM band to the TV band, there is a high possibility that the distortion is generated due to the high level signal from the FM band to TV band.
It has a second problem that the electrostatic energy is directly applied to the capacity Cagc and the diodes D.sub.1 and D.sub.2 when the antenna static electricity is induced. In this case, the capacitor Cagc and the diodes D.sub.1 and D.sub.2 are not always destroyed at once. However, since an element to restrict the electrostatic energy (e.g. inductance or resistance) is little or nil, the stress applied to the capacitor Cagc and the diodes D.sub.1 and D.sub.2 are not negligible in some cases. This will happen in the same manner even if the AGC elements are the transistors, for example, in place of the diodes.